The technology disclosed relates to writing or reading a pattern on a surface, such as in microlithography or inspection of mircrolithographic patterns. In particular, Applicant discloses systems recording or reading images by scanning sparse 2D point arrays or grids across the surface, e.g., multiple optical, electron or particle beams modulated in parallel. The scanning and repeated reading or writing creates a dense pixel or spot grid on the workpiece. The grid may be created by various arrays: arrays of light sources, e.g., laser or LED arrays, by lenslet arrays where each lenslet has its own modulator, by aperture plates for particle beams, or arrays of near-field emitters or mechanical probes. For reading systems, the point grid may be created by a sparse point matrix illumination and/or a detector array where each detector element sees only one isolated spot. The idea behind the use of large arrays is to improve throughput. However, the throughput does not scale with the array size, since above a certain size of arrays, previously known schemes fall into their own tracks and start repeating the same data over and over again. This application discloses methods to scan workpieces with large arrays while preserving the scaling of throughput proportional to array size, even for very large arrays, in fact, essentially without limits. Other advantages of the disclosed methods are greater flexibility in the choice of array size, workpiece grid, and stage parameters, and a dissolution of hardware signatures in the image, leading to a more ideal image in certain respects than with prior art.
Reading and writing of images can be done in a number of different architectures, e.g., multibeam raster scanning, assembly from small image elements, etc. In this disclosure we will only discuss one particular architecture: scanning the workpiece with sparse point arrays or matrices. This architecture is more and more important due to the increasing capability of microelectronic, photonics and MEMS technology, and the growing availability of large arrays of light and particle sources, modulators, near-field probes and detector elements. Currently, commercially available MEMS arrays may modulate more than 2 million light beams in parallel, at frame rates of more than 20 kHz. Likewise, large detector arrays have long existed as camera chips, and the size and speed is constantly being improved. The continued development of microelectronics, photonics and MEMS technology is likely to make large arrays of other types of elements available, such as near-field and mechanical probes, capacitive or Kelvin probes, magnetometers, lasers, LEDs, and LCD and electrooptic modulators. Arrays of charged particle blankers or massively parallel modulators for particle beams have been demonstrated by several groups. Electron beams also might be used.
The rationale behind large arrays with millions of elements is to get high throughput, but a closer study shows that it is difficult to use these massive arrays efficiently. The designer has to take into account issues of total field size, stage speed and overhead, and the limitations of frame rate in the modulator/detector and the light source. The result has so far not made full use of the inherent speed of the large devices already available.
FIG. 1a shows an example of a generic reading/writing system using a scanning sparse point array as known in the art. The following explanation uses a writing system as an example, and an alternative image-reading system may be extrapolated by the substitution of a detector array for the light source array.
The image-writing system in FIG. 1a creates an image 100 on a substrate 101 by a scanning motion 102 of the image of a point array 103. The point array has a sparse matrix or array of light source elements 104, e.g., VCSEL laser diodes, and each element is projected by means of some optics 106 onto the substrate to form image spots 105. The light source elements 104 are controlled (i.e., turned on or off) by a data path 108, according to an input description (not shown) of the pattern to be produced 100. The data path synchronizes the driving signals 107 sent to the source array 104 to the movement 102 of the substrate as measured by a position sensor 109.
In the example of FIG. 1, the stage is scanning with continuous motion and the light source array prints during a time short enough to freeze the motion and make an image of the turned-on light sources. Since the source array is sparse, the first image is not the desired pattern, but only an array of isolated dots. After the stage has moved a distance, a second dot pattern is exposed and so on. After a number of translations the desired pattern has been completely filled in 100.
Many modifications may be, or have been, contemplated: the projection lenses drawn in FIG. 1 may be replaced or supplemented with one or more lens arrays with one lens per spot. Or the distance between the sources and the substrate may be so short that no projection system is needed to form a spot on the substrate for each spot on the source array. The light source array may be a modulator array illuminated by a light source, and the modulators may be binary (on/off) or analog (many values, “gray scale”). The light may be visible, infrared, ultraviolet, deep ultraviolet, vacuum ultraviolet, extreme ultraviolet or even x-ray.
The same scheme is also useful for particle beams, e.g., using electrons, protons, ions or neutral atoms. The source array may then be an array of field or photoelectric emitters, or it may be an array of blankers (a so called aperture plate) illuminated from the back side, or it may be a reflection modulator for particles based on voltage contrast (e.g., similar to the modulator used by KLA-Tencor in the REBL, cf. U.S. Pat. No. 6,870,172 B1). For particles, the projection system may be electron optical lenses, either with a lens common to many points or with one lens per point in a lens array, a longitudinal magnetic field, or, again, in such close proximity that a projection system may not be needed at all.
A third possibility is that the source point array is an array of near-field probes, e.g., making a mechanical imprint, exposing by injection or extraction of electric charge to/from the surface, or measuring a property of the surface, e.g., the electrostatic potential or the magnetic field at the surface. An array of near-field optical probes based on field-concentration, plasmons and/or evanescent waves is another example of a possible source/detector point array.
The writing head (with the source array and/or the projection optics), or the substrate, or both may be physically moving to create a relative motion, or the image of the source array may be scanned by the optical means, e.g., by a galvanometer or polygon. With either light or particle optics the relative motion of the substrate can be continuous and frozen by a short exposure time, by stepping the substrate motion, and/or by letting the beams track the continuous substrate motion over a finite distance. In either case, the exposure of different spots may be simultaneous or they may be distributed in time, in which case the effect of timing and movement on the placement of the spots on the substrate has to be accounted for.
Different schemes to fill an area with images of isolated spots can be found in the prior art. The most obvious one is to use several rows of sources across the scanning direction and stagger the elements as is known in numerous patents, see FIGS. 1b-g. In the case of a light source and a modulator per element in the array, the light source may be continuous, e.g., a continuous laser or a laser that emits pulses close enough to be considered continuous. The stage is scanned with a low enough speed to let a modulator change state once per pixel in the grid on the workpiece. In recent patents by IMS Nanofabrication (U.S. Pat. No. 7,084,411 B2 and others) additional rows are added to provide redundancy for bad elements in the array. More elements are added in the same column and small-range scanning is used to let an element write only some pixels in the column on the workpiece to circumvent the speed limitation imposed by the highest practical switching speed of the modulator elements, here blankers in a massively parallel particle beam writer.
Related art is displayed in FIGS. 1b-g: Mark Davidson proposed in a paper in 1996 (Proc. SPIE, Vol. 3048, pp. 346-355) to rotate the array slightly so that each row in a 2D array traces a separate column. The light source was again continuous and the pixels were defined by switching the state of the modulators.
Kenneth Johnsson described a system (U.S. Pat. No. 6,133,986), FIG. 1c, with slanted spots in 1996, as did Ted Whitney several years before, FIG. 1b (US RE 33,931).
DNS has taken the scheme of Davidson one step further, as shown in FIG. 1d (U.S. Pat. No. 6,903,798).
In an invention intended to write on thermal resist, Gilad Almogy, of Applied Materials, has used a simple 2D interlace scheme in order to put every pixel non-adjacent to the last one, thereby avoiding the effect of heating of adjacent pixels, FIG. 1e (U.S. Pat. No. 6,897,941).
Ball Semiconductors gave the mathematics of the slanted scheme in 2004 (US2004/0004699), and ASML discussed using hexagonal grids in U.S. Pat. No. 7,230,677 (FIG. 1g).